The present invention relates generally to oscillating wave engines and refrigerators, and, more particularly, to thermoacoustic engines and refrigerators, including Stirling engines and refrigerators and their hybrids.
According to thermodynamic principles, acoustic power in a gasxe2x80x94a nonzero time average product of oscillating pressure and oscillating volume flow ratexe2x80x94is as valuable as other forms of work such as electrical power, rotating shaft power, and hydraulic power. For example, acoustic power can be used to produce refrigeration, such as in orifice pulse tube refrigerators; it can be used to produce electricity, via linear alternators; and it can be used to generate rotating shaft power, e.g., with a Wells turbine. Furthermore, acoustic power can be created from heat in a variety of heat engines such as Stirling engines and thermoacoustic engines.
Historically, Stirling""s hot-air engine of the early 19th century was the first heat engine to use oscillating pressure and oscillating volume flow rate in a gas in a sealed system, although the time-averaged product thereof was not called acoustic power. Since then, a variety of related engines and refrigerators have been developed, including Stirling refrigerators, Ericsson engines, orifice pulse-tube refrigerators, standing-wave thermoacoustic engines and refrigerators, free-piston Stirling engines and refrigerators, and thermoacoustic-Stirling hybrid engines and refrigerators. Combinations thereof, such as the Vuilleumier refrigerator and the thermoacoustically driven orifice pulse tube refrigerator, have provided heat-driven refrigeration.
Much of the evolution of this entire family of acoustic-power thermodynamic technologies has been driven by the search for higher efficiencies, greater reliabilities, and lower fabrication costs. FIGS. 1, 2, and 3 show some prior art engine examples.
FIG. 1 shows a free-piston Stirling engine 10 integrated with a linear alternator 12 to form a heat-driven electric generator. High-temperature heat, such as from a flame or from nuclear fuel, is added to the engine at the hot heat exchanger 14, ambient-temperature waste heat is removed from the engine at the ambient heat exchanger 16, and oscillations of the gas 18, piston 22, and displacer 24 are thereby encouraged. The oscillations of piston 22 cause permanent magnet 26 to oscillate through wire coil 28, thereby generating electrical power which is removed from the engine to be used elsewhere.
The conversion of heat to acoustic power occurs in regenerator 32, which is a solid matrix smoothly spanning the temperature difference between hot heat exchanger 14 and ambient heat exchanger 16 and containing small pores through which the gas oscillates. The pores must be small enough that the gas in them is in excellent local thermal contact with the solid matrix. Proper design of the dynamics of moving piston 22 and displacer 24, their gas springs 34/36, and gas 18 throughout the system causes the gas in the pores of regenerator 32 to move toward hot heat exchanger 14 while the pressure is high and toward ambient heat exchanger 16 while the pressure is low. The oscillating thermal expansion and contraction of the gas in regenerator 32, attending its oscillating motion along the temperature gradient in the pores, is therefore temporally phased with respect to the oscillating pressure so that the thermal expansion occurs while the pressure is high and the thermal contraction occurs while the pressure is low.
Those skilled in the art understand that another way to view the operation of the free-piston Stirling engine, and indeed all regenerator-based engines including all Stirling and traveling-wave engines, is that acoustic power flows into the ambient end of the regenerator, is amplified in the regenerator by a temperature gradient in the regenerator, and flows out of the hot end of the regenerator. Ideally, the heat exchangers at the ends of the regenerator are essentially transparent to this acoustic power flow. Ideally, the acoustic-power amplification factor in the regenerator is equal to the ratio of hot temperature to ambient temperature, both temperatures being measured in absolute units such as Kelvin.
In the free piston Stirling engine of FIG. 1, the acoustic power flowing out of the hot end of regenerator 32 is absorbed from the gas by the hot end of displacer 24 and immediately delivered to the gas at the opposite end of displacer 24. There, some of the acoustic power flows into the ambient end of regenerator 32 to provide the original acoustic power for amplification, and the rest is delivered by gas 18 to piston 22. Hence, circulating acoustic power flows through the regenerator from ambient to hot temperatures and is amplified therein.
FIG. 2 shows another regenerator-based engine: a thermoacoustic-Stirling hybrid engine delivering acoustic power to an unspecified load 42 (e.g., a linear alternator or any of the aforementioned refrigerators) to the right. High-temperature heat, such as from a flame, from nuclear fuel, or from ohmic heating, is added to the engine at hot heat exchanger 44, most of the ambient-temperature waste heat is removed from the engine at main ambient heat exchanger 46, and oscillations of the gas are thereby encouraged. The conversion of heat to acoustic power occurs in regenerator 48, which is structurally and functionally identical to that described in FIG. 1 for the free piston Stirling engine. Proper design of the acoustic network (including, principally, the feedback inertance 52 and compliance 54) causes the gas in the pores of regenerator 48 to move toward hot heat exchanger 44 while the pressure is high and toward main ambient heat exchanger 46 while the pressure is low. The oscillating thermal expansion and contraction of the gas in regenerator 48, attending its oscillating motion along the temperature gradient in the pores, is therefore temporally phased with respect to the oscillating pressure so that the thermal expansion occurs while the pressure is high and the thermal contraction occurs while the pressure is low.
As in the free piston Stirling engine, another way to view the operation of the thermoacoustic-Stirling hybrid engine is that acoustic power {dot over (E)}C flows into the ambient end of regenerator 48, is amplified by the temperature gradient in regenerator 48, and flows out of the hot end of regenerator 48. In FIG. 2, the acoustic power {dot over (E)}H flowing out of the hot end of the regenerator splits into two portions {dot over (E)}fb and {dot over (E)}res at the resonator junction 40, with the required amount {dot over (E)}C flowing into the ambient end of regenerator 48 to provide the original acoustic power for amplification, and the rest {dot over (E)}res being delivered to the right to the unspecified load 42. Hence, again, circulating acoustic power flows through regenerator 42 from ambient to hot and is amplified therein.
FIG. 3 shows a standing-wave thermoacoustic engine delivering acoustic power to an unspecified load 62 to the right. The standing-wave thermoacoustic engine creates acoustic power from heat in a somewhat different way than do regenerator-based engines such as those shown in FIGS. 1 and 2. High-temperature heat, such as from a flame, from nuclear fuel, or from ohmic heating, is added to the standing-wave thermoacoustic engine at hot heat exchanger 64, ambient-temperature waste heat is removed from the engine at the ambient heat exchanger 66, and oscillations of the gas are thereby encouraged. The conversion of heat to acoustic power occurs in stack 68, which is a solid matrix smoothly spanning the temperature difference between hot heat exchanger 64 and ambient heat exchanger 66 and containing pores through which the gas oscillates. The pores in stack 68 must be significantly larger than those in a regenerator operating under similar conditions, because excellent local thermal contact between the gas and the solid matrix is undesirable in a stack. Instead, deliberately imperfect thermal contact is necessary, and is typically provided by pore sizes of the same order, but slightly larger than, the thermal penetration depth in the gas at the operating frequency.
The oscillating thermal expansion and contraction of the gas in stack 68, attending its oscillating motion along the temperature gradient in the pores, is temporally phased with respect to the oscillating pressure so that the thermal expansion occurs while the pressure is high and the thermal contraction occurs while the pressure is low. However, this is achieved by fundamentally different circumstances than in the regenerators described for FIGS. 1 and 2. In stack 68, the temporal phasing between gas motion and gas pressure is such that the gas moves toward hot heat exchanger 64 while the pressure is rising (cf. xe2x80x9chighxe2x80x9d in a regenerator) and towards ambient heat exchanger 66 while the pressure is falling (cf. xe2x80x9clowxe2x80x9d in a regenerator). The deliberately imperfect thermal contact between the gas and the solid matrix of stack 68 is required in order to introduce a significant temporal phase shift, which does not exist in a regenerator, between gas motion and gas thermal expansion/contraction, so that the desired temporal phasing between oscillating pressure and oscillating thermal expansion/contraction is achieved.
Those skilled in the art understand that a stack does not rely on the presence of acoustic power to create more acoustic power. Instead, a stack requires that the temporal phasing between oscillating motion and oscillating pressure be substantially that of a standing wave, which, in principle, might carry no acoustic power, and the acoustic power flowing through the stack and/or created by the stack can flow in either direction, or can flow in both directions away from the center of the stack (as it does in the stack in FIG. 4 discussed below), without substantially altering the power-producing phenomena described above.
Those skilled in the art also understand that similar descriptions can be provided for regenerator-based and stack-based refrigerators. Similar to the regenerator-based engines, essential features of the regenerator-based refrigerators are that acoustic power must flow through the regenerator from ambient to cold, acoustic power is thereby attenuated, and the pores of the regenerator must be small enough to provide excellent thermal contact between the gas and the solid matrix. Similar to the stack-based engine, essential features of the stack-based refrigerator are that acoustic power can flow into the stack from either direction, acoustic power is absorbed in the stack, and the pores of the stack must be of a size that provides deliberately imperfect thermal contact between the gas and the solid matrix.
The term xe2x80x9cambientxe2x80x9d temperature refers to the temperature at which waste heat is rejected, and need not always be a temperature near ordinary room temperature. For example, a cryogenic refrigerator intended to liquefy hydrogen at 20 Kelvin might reject its waste heat to a liquid-nitrogen stream at 77 Kelvin; for the purposes of this cryogenic refrigerator, xe2x80x9cambientxe2x80x9d would be 77 Kelvin.
Note that, in all cases, a regenerator functions usefully only if it is sandwiched between two heat exchangers at different temperatures. Similarly, a stack functions usefully only if it is sandwiched between two heat exchangers at different temperatures. Hence, for brevity, the term a xe2x80x9cstack unitxe2x80x9d and a xe2x80x9cregenerator unitxe2x80x9d are used to describe such sandwiches of a stack between two heat exchangers and a regenerator between two heat exchangers, respectively. The building blocks for the invention described herein will therefore be regenerator units (engine or refrigerator) and stack units (engine or refrigerator).
None of the systems described above provides high efficiency and great reliability and low fabrication costs. For example, the free piston Stirling engine (FIG. 1) has high efficiency, but its moving parts (requiring tight seals between the piston and its surrounding cylinder and the displacer and its surrounding cylinder) compromise reliability and are responsible for high fabrication costs. The thermoacoustic-Stirling hybrid engine (FIG. 2) has high efficiency and high reliability, but the toroidal topology is needed for the circulation of acoustic power is responsible for high fabrication costs for two reasons. It is difficult to provide flexibility in the toroidal pressure vessel to accommodate the thermal expansion of the hot heat exchanger 44 and surrounding hot parts, and an adjustable jet pump 50 (shown in FIG. 2) must be provided to suppress Gedeon streaming around the torus, which would otherwise convect significant heat away from hot heat exchanger 44. Furthermore, there is published evidence that thermoacoustic-Stirling hybrid refrigerators can suffer from dramatic instability with respect to Gedeon streaming, so that an adjustable jet pump 50 would require continuous feedback-controlled adjustment. Finally, the stack-based standing-wave thermoacoustic engine (FIG. 3) is reliable and costs little to fabricate, but its efficiency is only about ⅔ that of the regenerator-based systems.
Accordingly, it is desirable to provide acoustic heat engines and refrigerators having, simultaneously, the high efficiency of regenerator-based systems, the low fabrication costs of no-moving-parts non-toroidal stack-based systems, and the reliability of no-moving-parts regenerator-based or stack-based systems.
Various advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
The present invention includes a thermoacoustic device with a resonator system defining at least one region of high specific acoustic impedance in an acoustic wave within the resonator system. A plurality of thermoacoustic units are cascaded together within the region of high specific acoustic impedance, where at least one of the thermoacoustic units is a regenerator unit.
In one aspect of the present invention, at least two regenerator units are connected in series within the region of high specific acoustic impedance. In another aspect, a plurality of regions of high specific impedance are placed along a common axis. In a particular embodiment, at least two of the plurality of regions of high specific impedance are separated by an acoustic side branch therebetween to provide an extended region of high specific acoustic impedance.